When ALT BRAKE IN USE is displayed on the WAP, which hydraulic system is powering the brakes?

When "ALT BRAKE IN USE" is displayed on the WAP (Warning and Advisory Panel), it indicates that the alternate braking system is active. In the C-17, the alternate braking system is powered by the No. 3 hydraulic system. This is important because the primary braking system relies on a different hydraulic system (specifically the No. 1 and No. 2 systems) for normal operation. The use of the alternate brake system could be due to a failure or malfunction in the primary braking systems, necessitating a switch to ensure safe landing and ground operations. Understanding this relationship between the warning displayed and the hydraulic systems strengthens the pilot's ability to respond appropriately in various scenarios involving hydraulic failures.